Electrolytic production of uranium powder



be- 28, 1954 w. c. LILLIENDAHL ET Al., 2,690,42

ELECTROLYTIC PRODUCTION OF' URANIUM POWDER Filed March 6, 1943 2Sheets-Sheet 1 5em- 23, 1954 vv. c. LILI IENDAHL ETAL 2,690,42

ELECTROLYTIC PRODUCTION OF' URANIUM POWDER 2 Sheets-Sheet 2 Filed March6, 1943 f M ATTORNEY Patented Sept. 28, 1954 ELEo'rnoLr'rrc PnoDUoTIoN0F URANM POWDER William C. Lilliendahl, Mountain Lakes, DonaldWrouglito'n, East Orange, RudolphV Nagy, Bloomfield, and John W. Marden,East Grange, N. J., assignors to the United States of Americaas reprcsentegl` by the United States Atomic Energy Commission Application March6, 1943, Serial No. 478,270

3 Claims.

This invention relates to the manufacture of uranium, and moreparticularly to the com-mercial p-roduction of said material in powderedform.

The principal object of our invention, generally considered, is toproduce uranium of exceedingly high purity on a commercial scale.

Another object of our invention is the large scale production of anoxygen-free' halide `salt". of uranium containing no undesirableimpurities,

A further object of our invention is the electrolysis of a bath composedof a pure' halide of uranium fused in a mixture of chlorides held atsuch a temperature that the metal uranium' is deposited as a coarsepowder on a cathode of refractory material such as molybdenum.

A still further object of our invention is the removal of theuranium-containing deposits from the cathode and treatment thereof' tocon'- serve the uranium powder produced.

Other objects and advantages of the invention', relating to theparticular arrangement and construction of the various parts, willbecome apparent as the. description proceeds'.

In the drawing illustrating ourV invention:

Fig. 1 is a flow diagram showing how potassiurn uranous fluoride isformed on a commercial scale and then washed, dried, yand electrolyzedto produce. uranium powderA embeddedA in other' electrolyte material;and' how said electrolyz'ed material is removed from the cathode,crushed, ground, washed in a tumbling barrel, ground, again treated in atumbling barrel, filtered, and

dried in vacuum, after which the metal powder may be pressed, andsintered' and/or fused to produce' the finishedw product.

Fig. 2 is a vertical sectional View illustrating apparatus forintroducing green salt or other' uranium compound to be electrolyzed,into the molten bath below" the surface thereof.

Fig. 3 is a View corresponding to Fig; 2 butV showing an alternativeapparatus for introducing pressed blocks of green salt beneath thevsurface ofl the' bath.

Fig; 4 is a View correspondingv to Fig. 3 but showing a furtheralternative apparatus, Where-f by green saltl may be introduced as asteady stream beneath the surface.

Fig.' 5 illustrates another embodiment for introducing green salt intothe bath,. while prdshowing anordinary' type of central electrodetrode,alternative to that shown in Fig. 1, in a fused bath held in itsassociated Crucible.

Fig. 8 is a View corresponding to Fig. '7, but

functioning as a cathode, with a modified form of crucible associatedtherewith;

Fig. 9 is a View corresponding to Fig. 8 but showing anothermodification for the same 91,1.1131056- Fis.'- 10 is a' transverseSectional View' Qn the line X-:X of Fig. 9', in the direction of theatr-4 rows; and Y Fig. l1 is a' vertical sectional View of a watercooled' container for smothering coatedcathodes in salt. t

Making green Salt In accordance' with our invention we rst preparerpotassium uranous fluoride, KUFs (called green $3145, and Wlflfh uraniumiS .tetra- Valerlt.) by a photoghanigal; reduction, prefer-- ably usingsunlight, of a solution containing a uranium salt such asthe nitrate (inwhich the uranium has avalency of six), potassium fluoride, hydrouoricacid, and a mougins agent Such as formic acid or sucrose The potassiumuranf ous'iiuoride precipitatedras a heavy green salt and collects atthe bottom of the vessel from which the liquor may be .dcanted and thedesired product removed, filtered, washed, and

dried to yield a salt suitable for the yc electrolysis operation.

The reducing agentV used for the production of the KUF5 may be formicacid, sucrose, oxalic acidI or other suitable, preferably organic,reducing. agen-t. Thel equation when using form-ic acid is `as follows:

agents. the production of potassium uranous fluoride;

When oxalic acid is used as a reducing agent,

A potassium uranous fluoride is produced in accord?- ance with thefollowing. equation:

It will, of course be u ndersGoodthat the `above equations are merelyillustrative of reducing actions for the production of potassium uranousiluoride.

Sucrcse is preferred as the reducing agent, as the rate of precipitationof potassium uranous fluoride is increased by more than over thatobtained with formic acid. However, there is more discoloration whenusing sucrose, which discoloration is of course undesirable, asexcluding some of the light needed for reduction.

In more detail, our process is carried out in the trays H, I2, i3, lll,l5, i6 and Il.

The trays illustrated in the drawing are in tended to be 10 X 12 feet inplan and lled to a depth of 4". This would make approximately 1080liters per tray. The reagents required per tray are as follows:

lar operation) and the desired quantities of potassium nuoride,hydrofluoric acid, and a reducing agent such as sucrose dissolvedtherein. Uranium nitrate is dissolved in its own weight of Water and theconcentrated solution added to the liquor in the trays. It isundesirable to add the concentrated reagents to the nitrate solu tion orto dissolve the solid nitrate in the solution of the other materials.

On cool days a yellow precipitate or scale is obtained, which is,however, soluble in hot water, with conversion to nitrate solution andKUF5. It is necessary, therefore, in case this salt separates, todissolve it by agitating it in hot water and then filtering andproceeding as directed for potassium uranous fluoride.

When the reaction has reached the desired The amount of potassiumuranous.` fluoride, which will be called for convenience green salt,which is produced per day is dependent upon the amount of sunlight.

Two days of sunlight in the spring and summer months at New Jerseylatitudes are required for conversion of 90% of the nitrate to potassiumuranous fluoride. A good sunny day, using the equipment disclosed,should produce about 150 grams of green salt per sq. ft. of tray area.This makes it possible to produce from 200 to 400 lbs. of green salt perday, depending on the ultra-violet intensity of the sunlight. It hasbeen determined that all of the ultra-violet component of the sunlightis utilized in the precipitation of the green salt as well as visiblecomponents including up to approximately 4300 Angstrom units.

The actual quantities and ratios of the various reagents used for theproduction of KUFS may vary, depending upon the desired method ofhandling, and weather conditions. For example. in cold weather, it maybe desirable to reduce the charge shown in the above table by as much as50% in order to reduce the amount of the yellow scale formed.

While other halides of tetravalent uranium such as "GF4 or UCl4 may beused in the electrolysis process described below, unless they are verypure the preparation of KUF5 in the manner described has certainadvantages when uranium metal of high purity is desired. Theprecipitation of KURS in the manner described results in the removal ofcertain impurities which are present in the normal commercial grade ofuranyl nitrate, resulting in a potassium uranous fluoride of high puritywhich in turn may be electrolyzed as hereinafter described to a verypure metal. The process is thus of great value when halides oftetravalent uranium of the desired chemical purity are not availablecommercially.

Other uranous fluorides such as lNaUF5 may be produced by a processsimilar to that described by changing the proper reagents, assubstituting sodium fluoride for potassium uoride.

The trays are first partially lled with water (or with supernatantliquor from a previous simidegree of completion, the supernatant liquidis decanted into the next lower tray and the precipitated potassiumuranous fluoride washed from the trays. The suspension is passed througha filter i8 and washed with distilled water and alcohol. In order toincrease the rate of iiltration a vacuum connection 2i is desirably madeto the container 22 which holds the ilter i8. The salt is then dried inair, as in an oven 23, at about C. Water I9 from filtration of greensalt is discarded.

In accordance with the drawing it will be seen that we propose to startwith full charges of the green-salt-producing bath in trays Il to ld,inclusive, while allowing the uranium content to gradually deplete intrays l5, I0 and Il by failure to add any more uranium nitrate to thesetrays. After recharging the sugar-containing solutions four times, it isconsidered necessary to discard the entire batch due to discoloration ofthe oxidized reducing agent.

The solution, depleted by precipitation of potassium uranous Iiuoride,contains some potassium nitrate and considerable hydrofluoric acid andpotassium fluoride. To the trays containing uranium depleted solutions,that is, those numbered l2 to ld, inclusive, which are to be maintainedup to strength and which successively receive the solutions from thepreceding ones of lower numbers, one mol of formic acid, cr 1/6 mol ofsucrose, together with 3 mols of potassium fluoride, and 2 mols ofhydroiiuoric acid are added for each mol of uranyl nitrate. This formulahas proven satisfactory but we do not wish to be limited to theseproportions.

As an alternative, di-potassium uranous fluoride, KzUFe, produced by aprocess described in the copending application, Serial No. 478,271 filedMarch 6, 1943, by John W. Marden and Rudolf Nagy, may be used in placeof potassium uranous fluoride. The metal produced from this salt is inevery way comparable with that produced from KUFs and UF4.

Electrolysis of "green salt The electrolysis of green salt to produceuranium powder may be carried out in furnace lll consisting ofy agraphite crucble 25 and a. molybdenum electro-de 2B. Thecrucible andelectrode are connected toa source of direct current 2-1 so that thecrucible is the anode and the` electrode 26' theA cathode. Means may beprovided for heating the contained electrolyte, other than by theelectrolysis currentl between the electrodes, such as resistancewindings about the Crucible (not shown). I-Iowever, we have found thatthe desired results may be obtained by heating the electrolytel with theelectrolysis current only.

The electrolysis process consists of fusing a. mixture of sodium andcalcium chlorides' in acrucible 25, adding toA said fused mixture thedesired proportion of green salt, andelectrolyzing the bath until theuranium is depositedonr the'A cathode. Using a batteryl` of fivefurnaces withv crucibles 9l inside diameter and 21" deep, with aneffective volume of; approximately 1300 cu; on three eight-hour shifts,approximately 250= lbs. oi-v metal powder per day may be pro-- duced.For each furnace; 18001amperes of' our rent with electrode changes every60? minutes are required.

The crucbles are' chargedV with metal halidesalts; desirably initiallycharged with a mixture consisting of about 80% calcium chloride byweight and about 20% sodium chloride. A totalof 80 to 1'00 pounds isjrequired.

If the salts are not fused by external heating', an auxiliary or "dummy`electrode may be used. The use of a dummy electrode during chargingserves not only for the hea-ting but also for the removal of deleteriousimpurities in the bathsuch as iron and boron prior to electrolysis.Such; impurities are deposited on the dummy during the chargingoperation orvolatilized from the bath by the heating.

The' bathis heated to the temperature of4 approximately 950 C. by thepassage of current and the cruciblelled towithin 6" of the top. When allthe salt is melted about one poundof green salt is added to thecrucible. About liveminutes after the addition of green salt theoriginal electrode, now called the dummyf is removed.

About seven pounds of green salt are then added to the'bath and the meltallowed tostand: until the temperature of thel bathfalls to 900 C. AcleanV electrode is then introduced' into-L the bath and theelectrolysis carried to completion. Allowing the; meltl to stand Withoutelectrolysis serves to eliminate any volatile dele-- terious materials,such asboron which passesoff as a halide or mixtureof halides, from thebathprior to electrolysis.

The amount of uraniumi salts initiallyv added to@ the fused chloridesshouldbe of the order of: to 3% of theztotal weightof the chlorides; inthe crucible. The amount. of uranium saltsl added after the dummycathode is removed from the.: crucible should be of the order of 5%'60,15% of the total weight of the chlorides in the crucible.

One batch of.V metal. powder made from several iirst electrodescontainedmore than parts perV million of' boron. Powder made fromcorresponding second electrodes. averaged about part per million. Powdermade by similar operations, except that one electrodeV was'used` forboth thev charging and the electrolysisaveraged I tor 3* partspermillionofv boron. Forcertain pur'- poses a minimum boron content isdesirable;

It external heat has initially been applied to-y f-use the bath; theintensity thereof: mustbe:

gas.

reduced while the: cell? is in. operation because of Ithe energyvdissipated byV the electrolysis current, asthetemperaturer mustbe hel-dvat 900LT Cl. i25` C. throughout aA run. Low temperatures result in largedeposits of salts.- on the electrodes but. low yields because of thefineness of deposited powder.

The ideadisclosed in- Fig. l. is to introducethe green salt into thefused bath by throwing it on top of the surfaces of said bath.Y It" thesalt. is potassium uranous` iiuoride'- or uranous-fluoridef, it mayoxidize. tol` some extent before becoming completely dissolved. Once.`dissolvedit ispro.- tected by the fusedbath and no oxidation seems. totake place. When the salt can beY plunged directly belowv the surface itis heavier than the calcium chloride or sodiuml chloride bath andi sinksto the bottom where it is: dissolved withoutoxidation.

In accordance with- Fig. 2': we propose. lio-introduce the dry' saltbelow the surface ofthe bath by weighing the desired amount into.v a-utube d'4 with a hinged cover' l5` at its` lower end and. initiallyclosed, as shownl infull lines. A plunger 46 is inserted into thetubewith the green salt. or other compound to be electrol-yzedf, placedin. the' bath 28a in the crucible 25e. and said salt expelled beneaththesurface after dropping theA cover 45, as by operating the rod 41connected' to a crank 481 thereon. We have simplified the showing byomitting the electrode.

Fig. 3 shows alternative apparatus for introducing "green salt in the.form of a pressed'. mass 49 beneath the surface of the electrolyte. bath20h in crucible-25b, as by holding it inf tongs: 51 and forcing itduicl'ilyY below the surface.V

In L we have shown furtherv alternative apparatus, for introducing"greensalt by means. of a small screw type conveyor 52 operating in acylindrical extension 5-3L on the bottom of a hopper 54contai'ning' thedesired quantity of' salt whichY is introduced into the bath- 28 fromlthe. lower open end- 55 of sai'df extension 53. The. screw conveyormember 52 may be operated through suitable gearing' by means of'anelectric motor or other' power means 56; In all'. forms. of' ourinvention illustrated in- Figs. 2, 3f. andl 4 the parts of the apparatuswhich. come in con tact with the fused bath are desirably made ofmolybedenumv or some such material in order to avoid contaminating saidbath.

Still anothermeans of introducing green salt" Without oxidation is toplacca cover` 51 of asbestos, or other halide-resistive insulatingmaterial, over the Crucible 215idw and flow inert or protective: gasYsuch as argon or hydrogen, onto the surface vof the bath as by means ofa valve-controlled pipe58; to prevent oxidation of theuranium salts ormetal during the process of introducing the saltv from the bottom ofhopper 59. Holes Pr'l` may be provided in thel cover 5'!Y to= allow forthe escape of gasgenerated during the-electrolysis operation and aslvventsvfor the protectivey A valve-controlled? branch` 62 ofthe pipe`581-mig-ht servey as ameans forI introducing gasv to control or expeditethe flow ofthe salt from. the hopperr 59. Another valveecontrolledfbranch. 63 might be employed to keep a protective at-l mosphere overthev top of the salt, a coverV 64 with a valve-controlled vent' piper65being in. suchl instance provided for the hopper 591.

The; cathodeI 2B is? lowered into thea bath tm within 2 or the Cruciblebottom and the; elec trolysis: currentv of about. 180,05 amper-es;started and maintained'- for. aboutl 60 inmates..

The furnaces or crucibles previously described for fused saltelectrolysis, in accordance with our invention, are of the single unittype, that is, the same Crucible is set in insulation and a singleelectrode placed in the center thereof. Any number of these furnaces maybe operated together in series or in parallel, but each is an individualunit. When an electrode is changed in a single unit type, the furnacemust be shut olf or shunted out. In very large installations suchconstruction is not suitable 1because the electrode becomes very largeand the cathode to Crucible spacing is too great. Handling the electrodethen becomes a problem and the voltage required to operate the furnaceis increased.

For operations on a large scale Where the metallic deposit is removed onthe cathode, a furnace with several cells built together, as illustratedin plan in Fig. 6, is desirable. The crucible 25e, which also serves asthe anode, in this instance is elongated and provided with baffles orpartitions 66, said baiiles projecting alternately from the oppositelong sides 73 and 76 of the Crucible. As in the previous instance, asuitable conductive material, such as carbon or graphite, must be used.An electrode 26e to function as a cathode is suspended in eachcompartment, except the first or melting compartment 6l, and allconnected together, as illustrated, so that the entire unit behaves asseveral cells connected in parallel.

The operation of such apparatus may be to charge a substantial quantityof the salts to make up the fused bath each time the uranium salts to beelectrolyzed are added. These salts may be charged into the endcompartment 6l, where they are melted, as by alternating current betweendummy electrodes S8 and 69. The molten salt will then flow into section'l l, through space l2 between partition @t and the long wall T3 of theassembly toward which it projects. From section ll it would flow on tosection 1d, through space 'l5 between the next partition 66 and the longwall 'it toward which it projects. From thence it would ilow on to theremaining sections or compartmentsI in a similar manner.

Uranium salt to be electrolyzed is added to each section except the rstand last, from time to time. In the last section no uranium salt isadded, this being used to take out the last trace of uranium before thesalts run to the overflow pipe ll. The electrodes may be removed one ata time whenever sufficient deposit has been collected. Moinentaryinterruption of the current in one section does not materially effectthe operation of the battery as a whole, as the other electrodes wouldcarry the eXtra current for a short while. Such a furnace isparticularly useful for operations where a large amount of makeup bathsalts must be used. It avoids loss of uranium in the discharge and makesthe process substantially continuous. Current limiting devices may beincorporated for use with the arrangement. Especially where it is notnecessary to recharge the bath, the rst section designated 6l may alsobe used for electrolysis like one of the single unit type furnacespreviously described.

Referring now to the embodiment of our invention illustrated in Fig. 7,there is shown a crucible 25f, which may be like a single unit crucible25 or a multiple unit crucible 25e, containing a fused salt electrolytebath 28f and in which is suspended a special type of electrode 26f tofunction as a cathode. The reason for the special type illustrated isthat while rnolybv` denum is at present the most suitable metal forthecathode, it is heavy and diilicult to fabricate. For best results such acathode should have a large diameter and surface area, Without eX-cessive cross sectional mass. A hollow cylinder satisfies theseconditions, but the fabrication of such a cylinder of sufficiently largedimensions out of molybdenum is difficult.

It is. therefore, desirable to construct a cathode with a surface ofmolybdenum on a base metal which may be readily shaped as desired. Thincylinders of molybdenum, such as might be constructed of sheet metal, donot have sufficient mass to conduct the heavy currents used inelectrolysis furnaces. In accordance with our invention we, therefore,propose to make the electrode 26f as a hollow cylinder 13 of steel orother suitable metal, closed at its lower end as indicated at 19, andcoated or sheathed from the bottom to above the top surface of theelectrolyte bath With molybdenum 8l. The molybdenum is desirably appliedas a continuous sheet or cup over the core '18, or as a ribbon or wireWound helically on the tube 18, so that the adjacent edges thereof abutand are thereafter desirably welded together so as to form, in effect, acontinuous molybdenum surface. Such an electrode has the advantage of alarge molybdenum surface, light weight, and a sufiicient but notexcessive cross sectional area. Covered electrodes of the type mentionedabove are disclosed and claimed in the copending application of WilliamC. Lilliendahl and Donald Wroughton, Serial No. 517,016, led January 5,1944, now abandoned.

rl'he assembled electrode 26f may be supported in a head forming aflange 82 from which a pipe 83 extends and connects with a port 8d tothe interior of the cylinder 73. The upper end of the cylinder is closedby a cap 35 carrying a pipe 86 of steel or other suitable material,which extends down to near the bottom of the cylinder 1S, and throughwhich air or other fluid may be introduced to said cylinder for coolingthe electrode when desired, as during withdrawal from the bath, in orderto minimize oxidation of the deposit. The cooling fluid would pass downthe pipe E6 and return between the pipe and the cylinder 18 through theport tl and out of the pipe 83, thereby effecting the desired coolingaction.

When using a cathode in the form of thin strips, a cylinder, orrectangular rod of molybdenum, it is found that When operated in aCrucible of the design shown in Figs. 1 to 7, inclusive, the deposit isnon-uniform. That is, the rate of deposition at the top portion of theelectrode, or near the surface of the bath, is faster than at the lowerportion. This results in a cone-shaped deposit, as shown in Fig. 1, onthe electrode 2t. This condition of unequal distribution is mostpronounced in electrodes of small cross sectional area.

Such a deposition is objectionable because it has a tendency to peel dueto the Weight of the deposit formed and sometimes results in loss ofdeposit. The formation of such deposits also reduces the electrolyzedmetal weight per unit area of cathode surface, which in turn limits thecharge per run of material electrolyzed, thus reducing the yield per manhour of operation. The condition appears to be due to the relativelylarge voltage drop in the electrode. In operation, this may be of anorder of magnitude of one to two volts.

As electrolysis starts, the voltage drop across the bath to the lowerportion of the cathode is less than at the top, which results inincreased deposit at the top of the electrode. This may also be due toconvection currents of the electrolyte. As electrolysis proceeds, thedeposit builds up at the top of the bath so that the resistance at thatpoint decreases, resulting in further increase in the rate of depositionat the top of the electrode at the expense of the lower portion.

This condition of unequal deposition may be prevented by tapering orforming the crucible 25g with an inverted frusto-conical interior, sothat the current along the cathode 26g is uniform throughout thesubmerged length, as shown in Fig. 8, or by having the cathode 25h flaredownwardly, or with its lowerportion truste-conical, in a crucible 25hof uniform cross section throughout its height, as shown in Figs. 9 and10, or by tapering the crucible to some extent and aring the cathode tosome extent, whereby the distance between the cathode surface and thecrucble surface steadily decreases from the top of the bath to thebottom of the cathode, the rate of decrease being sufficient to balancethe drop along the cathode, in order to equalize the current betweencathode and crucible and result in uniform thickness of deposit, asillustrated in Figs. 8, 9 and 10.

The cathode 26h, illustrated in Figs. 9 and 10, may be formed of twomolybdenum plates, trapezoidal in shape, with their widened portionsbulged or formed as hollow anged half-cones 8'! and 88, and thenconnected together as by molybdenum bolts 89. Current leads 9i areprovided for introducing the electrolysis current to the electrode. Whenthe electrolysis operation is completed the electrode may be withdrawnfrom the bath as usual, and after cooling the plates separated byremoving the bolts, thereby facilitating the removal of the deposit fromboth inside and outside. The upper bolts 80 (Fig. 9) are not immersed inthe bath and are easily removed. However the lower bolts acquire acoating during electrolysis that must be removed before disassembly ofthe electrode plates. The shape illustrated is not only conducive to auniform deposit, but is more eifective for preventing any slippage ofthe deposit by virtue of the vertical component of the pressure betweenthe deposit and the cathode, acting to support said deposit. Taperedelectrodes of the type mentioned above are disclosed and claimed in thecopending application of Towson Price, Serial No. 510,557, filedNovember 16. 1943, now abandoned.

Operation of the electrolyte bath at about 900 C. is an improvement overprior electrolysis processes in which operation was effected at lowertemperatures, in that the proportion of metal iines is decreased and acoarser uranium powder produced, which is less pyrophoric.

If a hollow electrode, as disclosed in connection with Fig. 7, isemployed, it is also desirable in accomplishing the above results toinject a rapid stream of air through it during the operation of raisingsaid electrode from the bath, thus assisting in the solidication ofsalts on the cathode to form a protective layer on the removed 10approximately 100 C., it has been found that said metal does not havethe pyrophoric properties which are present in powders when baths areoperated at 800 C.

The melting point of uranium is relatively low, that is approximately1150 C. With baths operating at 875 C. to 950 C., the actual temperatureat the cathode is materially aifected by current density and cathodepotential, which is in turn affected by bath composition. It has beendiscovered that, by a proper control of the variables, uranium metal isproduced on the cathode in such a compact form that the deposit can beremoved, washed with water, and the coherent metalized, sintered, massfused in vacuo, thereby eliminating the necessity of the washing andgrinding operations later fully disclosed.

The electrochemical equivalent of' tetravalent uranium is 2.22 grams perampere hour. The 7 pounds of green salt contain 2020 grams of uranium,which would require 910 ampere hours at 100% eiciency. In practice,however, approximately 1800 ampere hours are required for 7 pounds ofgreen salt. The current efflciency does not appear to be appreciablyaffected by current density at the cathode. The most convenientcombination of current and time which will yield 1800 ampere hours forthat size charge can be used, but the current strength must be thatrequired for keeping the furnace at the desired temperature.

At the completion of the run, the cathode is desirably taken quicklyfrom the bath for removal of the deposit. It is then preferablyimmediately placed in a tall cylindrical vessel 92 with a funnel shapedtop 93, as shown in Fig. 11, which is then at once filled with dry salt,such as dry sodium chloride, anhydrous calcium chloride, or similarmaterial 94. The salt is partly melted on the surface of the electrode,sealing the metal particles and preventing oxidation. If a hollowair-cooled electrode 2Ef, as shown in Fig. 7, is used, then it may notbe necessary to use the enclosing receptacles 95 containing coolingmeans, such as water 96, which is introduced by pipe 97 and overflowsfrom pipe 98.

However, for the ordinary type of electrode 26j shown in Fig. 1l, theexternal cooling means is desirable as it facilitates the operation byeffecting a quicker cooling. The purpose of this treatment is to preventoxidation while the deposit cools. Other methods of accomplishing theresult, such as quickly redipping in the bath after partial cooling, orpouring some of the electrolyte over the withdrawn cathode after partialcooling, have also been used successfully. Use of this improvedtechnique of salt smothering has enabled us to substantially increasethe average yield.

At the end of an electrolysis run, enough salt should be removed fromthe bath so that 12 to 15 pounds (that is about 15%, of the total chargeof fresh salt mix), of calcium chloride and 10% sodium chloride, may beadded to the crucible. Some salt is always removed on the electrode andthe rest may be dipped from the molten bath, as by means of a suitablecup. The furnace is then charged, preferably using the dummy electrodeas described, and the operation repeated.`

We do not wish our invention to be limited by the specific dimensions ofthe furnaces as given above. Other sizes of furnaces have been used forcarrying out the same process with good results. Also bath-temperaturesbetween 750 C.

and 950 C. will yield uranium metal, although 900 C. is the preferredtemperature since, other factors being equal, operation at 900 C. hasbeen shown to result in an increase in yield of usable metal powder.

The bath composition may vary considerably from the original charge, i.e. 80% CaCl2-20% NaCl, with good results. The bath described isdesirable because it has a sufficiently low melting point and requires aminimum amount of recharging to preserve fluidity.

Furthermore, halides of tetravalent uranium other than KUFs, such as UE;and UCli, may be electrolyzed in the same bath. Uranium tetrailuoride,(UFi) for example, has the advantage that it gives a better materialyield than KDF-5, based upon the uranium content. Whenever it can beobtained in the required degree of purity it then becomes the preferreduranium salt. When using UF4, it is desirable to recharge the furnaceswith an 80% Camz-20% NaCl mixture rather than with the 90% CaCl2-10%NaCl mixture used with KUFs. This compensates for the potassium contentof the KUF5.

In all cases where the fluoride salts of uranium are used forelectrolysis, it is necessary to discard a certain fraction of the bathat frequent intervals to remove part of the nuorides from the bath. Forbest results, the calcium fluoride content should not exceed 30%,although good yields of metal have been obtained from baths containingas much as 40% CaFz. A bath which gave very good results, for example,was composed of 18.7% CaFz, 60.8% CaClz, and 10.5% NaOH-% KCl after allthe uranium salts had been electrolyzed.

However, when operating the electrolyte bath at about 900 C., theelectrolyte is very fluid even with high percentages of calciumfluoride. We have, therefore, been able to use the same bathcontinuously for more than times with no removal of electrolyte, as bydipping. Salts, generally calcium chloride and sodium chloride, as inthe ratio of 4 to 1, are added but only to compensate for what is lostby evaporation and removal on the electrode.

Baths operated continuously, as described, are considered superior toperiodically replaced baths in the following respects:

( 1) Contamination by iron is minimized.

(2) The amount of calcium chloride and sodium chloride used in theprocess is substantially reduced.

(3) The dangers atending dipping are eliminated.

(4) The power necessary for heating the recharged bath is saved.

(5) The oxides produced in the bath, due to heating by electrolysiscurrent are eliminated. Such oxides increase the melting point, andinduce slipping and poor yields of metal.

(6) More runs per day can be made.

(7) Larger charges of uranium salts can be used, thus increasing theelectrical efliciency.

(8) More than one electrode can be used in a bath with alternateremovals.

As an alternative, to the use of the dummy electrode, the salt chargemay be melted in an auxiliary furnace and poured into the electrolysiscell when molten. In such cases, a dummy electrode need not be used ifthe salts are of satisfactory chemical purity.

Conversion of electrode deposit The deposit on the electrode isconverted to pure metal powder by a process which consists essentiallyof crushing and grinding said deposits, and washing and rinsing thecrushed and ground material until the residue is of the desired finenessand substantially free of deleterious substances, such as salts from theelectrolysis bath and finely divided uranium metal and oxide.

The electrode deposit as removed from the bath consists of metal powderand solidified salts which serve to protect the metal from oxidation.The ratio of salt to metal powder depends on the temperature andcomposition of the bath, high temperature and low viscosity favoring alow ratio of salt to metal powder. Operating at 875 C. to 900 C., theratio of salt to metal is approximately 2 to l.

The handling of the cathode and its deposit after removal from the bathmay, for example,

be divided into several unit operations, which comprise:

l. Removal of deposit from the cathode, as by means of an air hammer 29or a cold chisel.

2. Coarse crushing to i mesh or nner, as by means of a jaw Crusher 3|.

3. Fine crushing, as by means of a roller crusher 32.

4. Water washing to remove soluble salts, as in a tumbling barrel 33.

5. Wet grinding to break up aggregates and assist in the removal ofinsoluble salts, as by means of disk grinder 34.

6. Further washing, which may include the use of acetic acid, to removesalts retained from previous operations, as in a tumbling barrel 35,followed by repeated decantation.

7. Filtration of metal powder followed by Washing with alcohol and ethylether, as in vacuum lters 3G.

8. Drying in vacuo and. preserving in an atmosphere of inert gas, as invacuum ovens 31.

For producing uranium of extreme purity, all operations should beconducted in equipment composed of non-contaminating material, such aswood or hard rubber, where practical, and the grinding and crushingequipment made of steel, to reduce the pick-up of foreign elements. Hardglass which might contain borates is to be particularly avoided andwhere the use of glass equipment is necessary, silica or lime glassshould be used. The steps previously noted will now be discussed indetail with respect to the practice for large scale production.

For handling deposits comprising 250-300 lbs.

of metal powder per day, a jaw crusher capable of handling 900 lbs. ofmaterial per day is required. The crusher should have an efectve jawopening of approximately 1A".

After passing through the coarse jaw crusher 3i the material is passedthrough the roller crusher 32 set at approximately 0.100 and thentransferred to a wooden tumbling barrel 33, rotating at from 60 to 80 R.P. M., and provided with suitable mechanism for tipping to obtain rapidwashing by decantation. Sufiicient distilled water is added to dissolvethe soluble salts and the barrel rotated for a period of 20 minutes.Considerable heat is evolved during this operation, due to the solutionof the calcium chloride contained in the deposit, and it is necessary tocool, as by the addition of ice. The temperature of the solution shouldbe maintained below 30 C.

during this operation to avoid oxidation of the uranium powder.Approximately 8 liters of distilled Water are required per pound ofmetal pow- 13 der in the deposit, `to which-21m 3 lbs. of ice are added.

After completion of the tumbling, .the barrel is tipped and the metalsludge washed three times -by decantation, rotating two to three minutesbetween each operation. The wash vliquors should be recovered andsettled, as `the uranium metal nes and oxides should be reconverted tonitrate for subsequent precipitation as green salt in order to avoidloss of valuable material.

The metal sludge after removal from the tumbling vbarrel is run Wetthrough a .disk grinder 34 to break up aggregates ,and assist in furtherremoval of insoluble oxides and fluorides. A grinder capable of yieldingmaterial between 50 and 100 mesh is required for satisfactory removal ofthe insoluble materials. Particles coarser than 50 mesh interfere withsuccessful removal of impurities in the washing to follow.

The ground material is transferred to the rtating barrel 35 and washedwith water. After rotating for minutes, the barrel is tilted and thelight impurities poured off. This process, tumbling for 2 to 3 minutesand pouring off, is repeated with distilled water until the supernatantliquid is free from light material, such as CaF-z. In some cases a 2%solution of acetic acid may be used, in one of the early washes toassist in the process.

The metal powder is then transferred to a suction filter of the Buchnertype and the filtration facilitated by a connection to vacuum line 39.The metal on the iilter is desirably washed twice with 95% ethylalcohol, using suflicient reagent to cover the sludge and drainingbetween additions. The powder is then desirably washed twice with ethylether, acetone or other volatile solvent, and after partial drying isimmediately transferred to tared cans and placed in Vacuum ovens 3l.After drying in vacuum for eight hours, the ovens are lled with an inertgas such as nitrogen, carbon dioxide or argon, but preferably with thelatter, and the metal powder preserved therein until ready for thepressing operation. However, the long vacuum oven drying may be omitted,if desired, as the powder can be sufficiently dried on a suction filterafter washing with a volatile solvent. To reduce re hazard, the metalpowder should never be handled in lots larger than about 10 lbs.,especially after washing with ether or acetone.

Further operations may involve consolidating the metal powder in pressil and nally sintering and fusing, or otherwise heat treating, infurnaces 42, as in accordance with Patent No. 1,814,719.

From the foregoing it will be seen that we have devised an improvedmethod comprising rst purifying the raw material by transforming to anoxygen-free iiuori-de, while simultaneously avoiding contamination withdeleterious materials, particularly boron, and after electrolysispreventing subsequent contamination and oxidation, using only waterwhich is distilled, so that the uranium metal powder when nished issuitable for subsequent processing to form pure metal articles of thedesired shapes.

Although preferred embodiments of our ideas have been disclosed, it willbe understood that modiiications may be made within the broad spirit andscope of the invention.

We claim:

1. The method of producing pure uranium powder that comprisesintroducing a single uranous halide selected from the class consistingof potassium uranous fluoride, uranous fluoride and uranous chlorideinto a carbon crucible containing a fused bath consisting ofapproximately calcium chloride and 20% sodium chloride by weight andhaving a molybdenum element therein, the uranous halide being introducedin an amount vapproximating 5% to 15% by weight of the fused bath,electrolyzing at `a temperature of approximately 900 C. with theCrucible Vas anode andthe molybdenum element as cathode to deposituranium on said cathode, removing the coated cathode, permitting thecathode to cool, removing the deposit therefrom, and crushing, washingYand drying said deposit to produce pure uranium powder.

2. The method of producing pure uranium powder that comprisesintroducing potassium uranous fiu'oride into a carbon Cruciblecontaining a fused bath consisting of approximately 80% calcuimchloride, and 20% sodium chloride :by weight and having a molybdenumelement therein, the potassium uranous fluoride being introduced in anamount approximating 5% to 15% by weight of the fused bath,electrolyzing at a temperature of approximately 900 C., with thecrucible as anode and the molybdenum element as cathode to deposituranium on said cathode, removing the coated cathode, permitting thecathode to cool, removing the deposit therefrom, and crushing, washingand drying said deposit to produce pure uranium powder.

3. The method of producing pure uranium powder by electrolysis in acontinuously operating bath that comprises introducing potassium uranousfluoride into a carbon crucible containing a fused bath consisting ofapproximately 80% calcium chloride and 20% sodium chloride by weight andhaving a molybdenum element therein, the potassium uranous fluoridebeing introduced in an amount approximately 5% to 15% by Weight of thefused bath, electrolyzing at a temperature of approximately 900 C. withthe crucible as anode and the molybdenum element as cathode to deposituranium on said cathode, removing the coated cathode from the Crucible,removing a portion of said bath and refilling said bath to its originallevel with a mixture consisting of approximately calcium chloride and10% sodium chloride, the portion removed being such that the addition ofthe mixture in an amount approximating 15% by weight of the originalfused bath will restore the bath to its original level, introducing anew charge o-f potassium uranous fluoride into the refilled bath andelectrolyzing at a temperature of approximately. 900 C'.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 596,458 Inglis Dec. 28, 1897 618,575 Lyte lIran. 3l, 1899808,066 Borchers et al Dec. 26, 1905 850,376 Kugelgen et al Apr. 16,1907 1,077,894 Stevens Nov. 4, 1913 1,197,137 McNitt Sept. 5, 19161,255,197 Malm Feb. 5, 1918 1,534,319 Hoopes et al Apr. 21, 19251,646,784 Marden Oct. 25, 1927 1,814,719 Marden et al July 14, 19311,821,176 Driggs et al Sept. 1, 1931 1,826,806 Marden et al Oct. 13,1931 (Other references on following page) Number Number UNITED STATESPATENTS Name Date Driggs Dec. 8, 1931 Driggs Jan. 19, 1932 Driggs et alJune 7, 1932 Driggs Aug. 30, 1932 Eldridge June 23, 1936 Lyons Nov. 3,1936 Dandt et al Jan. 19, 1937 FOREIGN PATENTS Country Date France July30, 1906 "16 OTHER REFERENCES Driggs et al., article in Industrial andEngineering Chemistry, May 1930, pp. 516-519.

Smyth, Atomic Energy for Miliary Purposes, page 27, para. 2.27, August(1945).

Goodwin, Electrolytic Calcium in Proceedings of the AmericanPhilosophical Society, vol. 43, pages 383 and 384 (1904).

Croggins, Unit Processes in Organic Synthesis, published 1938 byMcGraw-Hill Book Co. Inc., page 604.

Morrow, Biochemical Laboratory Methods, published 1927 by John Wiley andSons Inc., page 191.

1. THE METHOD OF PRODUCING PURE URANIIUM POWDER THAT COMPRISES INTRODUCING A SINGLE URANOUS HALIDE SELECTED FROM THE GROUP CONSISTING OF POTASSIUM URANOUS FLUORIDE, URANOUS FLUORIDE AND URANOUS CHLORIDE INTO A CARBON CRUCIBLE CONTAINING A FUSED BATH CONSISTING OF APPROXIMATELY 80% CALCIUM CHLORIDE AND 20% SODIUM CHLORIDE BY WEIGHT AND HAVING A MOLYBDENUM ELEMENT THEREIN, THE URANOUS HALIDE BEING INTRODUCED IN AN AMOUNT APPROXIMATELY 5% TO 15% 